Physcomitrium is from Greek,
physcos, something inflated, and mitrion, a little cap, in
reference to the inflated calyptra. The genus is commonly found on bare soil in
weedy places and is characterized by more or less rostrate opercula,
eperistomate capsules, mitrate‑rostrate calyptrae, and short exothecial
cells. Gametophytically, the plants are indistinguishable from most of the
other confamilial genera. Physcomitrium does not appear to be
monophyletic, but rather the end point of a number of reduction series in the
Funariaceae. Based upon a consideration of columella form some species, like
the common North Temperate P. pyriforme (Hedw.) Hampe, are not all that
different from Funaria in having a large air space below the spore sac
which is traversed by trabeculae. The columella is relatively robust and is not
attached to the operculum at maturity. At another extreme are species in which
there is no air space subtending the spore sac and the capsules are weakly
systylius or the columella is attached to the operculum and falls with it at
maturity. The type of the genus, P. sphericum (Ludw.) Fürnr. of Europe
and northern Asia, falls into the latter category.

Plants small to medium‑sized, pale‑green, solitary to gregarious. Stems simple to irregularly and weakly branched, more or less acaulous to 1 cm tall, sclerodermis and central strand present. Leaves erect‑ to wide‑spreading, crispate or incurved when dry, obovate to spathulate to oblong‑obovate, obtuse, acute or acuminate, often concave; margins usually bluntly serrulate above, rarely entire throughout, plane throughout or rarely recurved below; costa slender to moderately stout, subpercurrent to short‑excurrent; cells oblong‑hexagonal or subquadrate to short‑rectangular, thin‑walled, smooth, toward the margins usually becoming more elongate and forming a border, basal cells becoming larger, laxer and thinner‑walled, alar cells not differentiated. Autoicous, rarely paroicous, polygamous, or synoicous. Setae very short to elongate, typically straight when moist, yellow to reddish brown, smooth. Capsules immersed to exserted, erect, broadly short‑oblong to hemispheric when operculate, cupulate to urceolate to pyriform or turbinate when empty, the mouth usually as wide as the urn, with or without a well developed neck; exothecial cells more or less isodiametric to short‑rectangular, less than 2:1, evenly thick‑walled to collenchymatous, typically becoming oblate at the mouth; stomata restricted to base of capsule or neck, superficial or immersed; opercula high‑conic to low‑ convex and apiculate or rostrate, cells in poorly defined straight rows; annuli usually of a single row of more or less quadrate, thin‑ to firm‑walled cells, persistent or fragmenting, rarely compound and revoluble; peristome none. Spores spherical to subreniform, brown, large, strongly ornamented. Calyptrae mitrate‑rostrate, only covering rostrum and less than half the capsule, falling before maturity.